Local regulations, as well as concerns about public safety and liability, require enterprises to provide employees and visitors with an effective means to reach a universal emergency number (e.g., 911 in the U.S., 000 in Australia, and 112 in the European Community) operator in an emergency. The call routes through the local central office, through a switch, to the appropriate Public Safety Answer Point (PSAP), where the call is answered. Each PSAP covers one city or one rural community. At the PSAP, emergency operators determine the nature of the emergency and contact the appropriate agency: typically police, fire, or emergency medical services. A single PSAP is typically responsible for an area covering several independent police and fire departments in the U.S.
With Enhanced 911 (E911), the calling party number, known as an Emergency Location Information Number (ELIN), is sent with the emergency call over Centralized Automatic Message Accounting (CAMA) trunks or via the calling number information element over Integrated Services Digital Network (ISDN) trunks. A suitably configured module at the PSAP uses the ELIN to lookup the caller's documented street address location from the Automatic Location Information (ALI) database.
To allow operators at the PSAP to call back a caller, IP communication systems use Public Switched Telephone Network (PSTN)-based methods to send an ELIN that identifies the telephone number for an extension from which the emergency call was dialed. If the extension number is not public, premise-based communications systems can be programmed to send an ELIN that is located nearby the calling extension. This is typically done by setting up the Local Area Network (LAN)-based telephony system itself, a media gateway, or a separate server, to automatically send a properly formatted number, such as the public telephone number. To deliver this information to the public emergency services network, the information can be encoded over analog CAMA (which requires an 8-digit ELIN), or over digital ISDN trunks (which require a 10-digit ELIN).
The above schemes assume that a calling party number or ELIN always corresponds to the street address in the ALI database. This assumption is not always true in practice. Adds, moves, and changes occur frequently in a dynamic enterprise communications environment. IP Telephony enables end users to relocate automatically their telephones to any location that can access the Wide Area Network (WAN). For example, H.323 IP telephone users can move telephones without notifying the system administrator of the move and Session Initiation Protocol (SIP) telephone users can use the same extension number at several different telephones simultaneously. If the users of such telephones dial a universal emergency number, emergency response personnel may go to the wrong physical location.
To address this problem, Cisco Systems™ has introduced the Emergency Responder™. The operation of the Emergency Responder will be discussed with reference to FIG. 1. A communications system comprises a switch 100, an emergency responder 104, a first Ethernet switch 108 in a first subnet 112 and having a plurality of ports 116a-n connected to a plurality of telephones 120-1 to -N, and a second Ethernet switch 122 in a second subnet 124 and also having a plurality of ports 128a-n, all of which are connected by a network 132. As used herein, a “subnetwork” or “subnet” refers to a network segment. The emergency responder 104, at predetermined time intervals, queries the switch 100 (or Ethernet switches directly) for new phone and user login registration events. In response to reported events, the emergency responder queries the various Ethernet switches in the network 132 to determine the location of the telephone and the user, based on the switch port to which the phone is attached. The location, which is based on the Open Systems Interconnect (OSI) Layer 2 (or Data Link Layer) port address, is then updated in an emergency responder location database (not shown). Thus, if telephone 120-2 is moved from port 116b of the first Ethernet switch 108 to port 128b of the second Ethernet switch 122 (as shown by the dotted lines), the emergency responder will, in response to periodic queries of the switch 100, determine that the telephone has moved. The responder then must query each and every Ethernet switch in the same subnetwork to determine to which port the telephone has been moved. When an emergency call is placed, the switch queries the emergency responder for location information associated with the call. In response to the query, the emergency responder locates and transmits the requested information to the switch, which then forwards the information along with the call. The location is the ELIN of a real or virtual phone that has a port close to the physical location of the port of the calling phone and has a street address that is known to be correctly entered in the ALI database. When an emergency call is dropped, any calls to the ELIN are forwarded automatically for a predetermined period of time to the device originating the dropped call.
The methodology employed by the Emergency Responder™ has been embodied in TSB-146, Telecommunications—IP Telephony Infrastructures—IP Telephony Support, Emergency Calling Service, formulated under the cognizance of TIA Subcommittee TR-41.4, IP Telephony Infrastructure and Interworking Standards (“TIA Standard”). The TIA Standard requires connectivity between the IP communication device (shown in FIG. 1 as the telephones), the switch/server (called “call agent or server” in the standard and shown in FIG. 1 as the switch, an adjunct called the “Location Information Server” (shown in FIG. 1 as the emergency responder), and Simple Network Management Protocol (SNMP) ports on every data switch (shown in FIG. 1 as the first and second Ethernet switches) in the same subnet as the IP communication device.
The approach of the Emergency Responder™ and the TIA Standard can have a number of drawbacks. For example, the emergency responder, due primarily to the time required to learn of the change from the switch (due to the polling interval), is unable to collect location information on a real time basis. It is thus possible that, if a telephone is moved to another ethernet switch and dials an emergency call before the emergency responder is able to ascertain the new location of the telephone, emergency personnel will be sent to an incorrect location (which in FIG. 1 is associated with the first subnet rather than the second subnet). The likelihood of this happening depends on the duration of the polling interval of the Ethernet switches by the responder 104. As will be appreciated, switch queries are done neither too frequently as the network will be flooded with traffic nor too infrequently as a user might make an emergency call before the emergency responder recognizes the user's IP telephone has moved. Because the emergency responder 104 is queried during an emergency call for location information, there is a possibility that ELIN information may not be provided to the emergency provider, that the emergency call will be delayed until the information is received, or that an incorrect ELIN (such as a default ELIN) may be provided. Not only is there a time delay in obtaining the information, namely the switch 100 must send a query through network 132 to the responder 104 and then the responder 104 must send a response through the network 132 to the switch 100 but also the location information is not accessible if the emergency responder cannot be contacted for some reason. For example, the emergency responder or a link on a communication path to/from the responder may malfunction.